It has been well accepted that the tumor microenvironment can significantly modulate growth rate, patterning and function of the vasculature. Our long term objective is to gain molecular understanding of the tumor microenvironment with the goal to suppress tumor angiogenesis. We have recently found that a specific pool of matrix metalloproteases can process VEGF-A severing the receptor-binding motif from the matrix-binding domain and releasing bioactive growth factor from matrix-bound VEGF. Interestingly, we also found that matrix-bound VEGF is able to phosphorylate VEGFR2 to the same degree as soluble VEGF. Nonetheless, bound and soluble VEGF initiate alternative modes of vascular expansion, namely while matrix bound VEGF leads to sprouting angiogenesis that favors tumor growth;soluble VEGF results in vascular hyperplasia and increased permeability. Thus, the relative abundance and composition of the extracellular matrix in tumors and its pool of proteases can dictate the vascular responses triggered by VEGF. Currently, our research is directed towards characterizing the mechanism that result in alternative morphogenetic events initiated by soluble and bound VEGF and their relevance to tumor growth and therapy. Preliminary data generated by our laboratory supports the hypothesis that: "The mode of VEGF ligand presentation alters VEGFR2 phosphorylation status and regulates differential cellular responses". This hypothesis will be further tested by three specific aims: (1) To define the signaling properties of soluble and bound VEGF and (2) To characterize the interactions of VEGF with the extracellular matrix and determine the biological relevance of MMP-mediated processing of this growth factor. Furthermore we will explore the significance of autocrine VEGF signaling to tumor growth. Based on our preliminary studies using a cre-lox approach, we predict that autocrine VEGF is critical to endothelial cell survival and can impose resistance to certain modalities of anti-angiogenic therapy. Thus, in Aim 3 we will investigate the function of autocrine VEGF signaling and its role in anti-angiogenic therapy. Angiogenesis is a complex phenomenon involving signals from endothelial cells and from the host tissue. Information from the experiments proposed in this application will further elucidate the influence of the tumor microenvironment in VEGF signaling. These data will aid in further refinement of therapeutic strategies to inhibit angiogenesis in tumors.